courgette/disassembler_elf_32.cc

   1 // Copyright 2013 The Chromium Authors. All rights reserved.
   2 // Use of this source code is governed by a BSD-style license that can be
   3 // found in the LICENSE file.
   4
   5 #include "courgette/disassembler_elf_32.h"
   6
   7 #include <algorithm>
   8 #include <string>
   9 #include <vector>
  10
  11 #include "base/basictypes.h"
  12 #include "base/logging.h"
  13 #include "base/memory/scoped_vector.h"
  14
  15 #include "courgette/assembly_program.h"
  16 #include "courgette/courgette.h"
  17 #include "courgette/encoded_program.h"
  18
  19 namespace courgette {
  20
  21 DisassemblerElf32::DisassemblerElf32(const void* start, size_t length)
  22   : Disassembler(start, length),
  23     header_(NULL),
  24     section_header_table_(NULL),
  25     section_header_table_size_(0),
  26     program_header_table_(NULL),
  27     program_header_table_size_(0),
  28     default_string_section_(NULL) {
  29 }
  30
  31 bool DisassemblerElf32::ParseHeader() {
  32   if (length() < sizeof(Elf32_Ehdr))
  33     return Bad("Too small");
  34
  35   header_ = (Elf32_Ehdr *)start();
  36
  37   // Have magic for elf header?
  38   if (header_->e_ident[0] != 0x7f ||
  39       header_->e_ident[1] != 'E' ||
  40       header_->e_ident[2] != 'L' ||
  41       header_->e_ident[3] != 'F')
  42     return Bad("No Magic Number");
  43
  44   if (header_->e_type != ET_EXEC &&
  45       header_->e_type != ET_DYN)
  46     return Bad("Not an executable file or shared library");
  47
  48   if (header_->e_machine != ElfEM())
  49     return Bad("Not a supported architecture");
  50
  51   if (header_->e_version != 1)
  52     return Bad("Unknown file version");
  53
  54   if (header_->e_shentsize != sizeof(Elf32_Shdr))
  55     return Bad("Unexpected section header size");
  56
  57   if (header_->e_shoff >= length())
  58     return Bad("Out of bounds section header table offset");
  59
  60   section_header_table_ = (Elf32_Shdr *)OffsetToPointer(header_->e_shoff);
  61   section_header_table_size_ = header_->e_shnum;
  62
  63   if ((header_->e_shoff + header_->e_shnum ) >= length())
  64     return Bad("Out of bounds section header table");
  65
  66   if (header_->e_phoff >= length())
  67     return Bad("Out of bounds program header table offset");
  68
  69   program_header_table_ = (Elf32_Phdr *)OffsetToPointer(header_->e_phoff);
  70   program_header_table_size_ = header_->e_phnum;
  71
  72   if ((header_->e_phoff + header_->e_phnum) >= length())
  73     return Bad("Out of bounds program header table");
  74
  75   default_string_section_ = (const char *)SectionBody((int)header_->e_shstrndx);
  76
  77   ReduceLength(DiscoverLength());
  78
  79   return Good();
  80 }
  81
  82 bool DisassemblerElf32::Disassemble(AssemblyProgram* target) {
  83   if (!ok())
  84     return false;
  85
  86   // The Image Base is always 0 for ELF Executables
  87   target->set_image_base(0);
  88
  89   if (!ParseAbs32Relocs())
  90     return false;
  91
  92   if (!ParseRel32RelocsFromSections())
  93     return false;
  94
  95   if (!ParseFile(target))
  96     return false;
  97
  98   target->DefaultAssignIndexes();
  99
 100   return true;
 101 }
 102
 103 uint32 DisassemblerElf32::DiscoverLength() {
 104   uint32 result = 0;
 105
 106   // Find the end of the last section
 107   for (int section_id = 0; section_id < SectionHeaderCount(); section_id++) {
 108     const Elf32_Shdr *section_header = SectionHeader(section_id);
 109
 110     if (section_header->sh_type == SHT_NOBITS)
 111       continue;
 112
 113     uint32 section_end = section_header->sh_offset + section_header->sh_size;
 114
 115     if (section_end > result)
 116       result = section_end;
 117   }
 118
 119   // Find the end of the last segment
 120   for (int i = 0; i < ProgramSegmentHeaderCount(); i++) {
 121     const Elf32_Phdr *segment_header = ProgramSegmentHeader(i);
 122
 123     uint32 segment_end = segment_header->p_offset + segment_header->p_filesz;
 124
 125     if (segment_end > result)
 126       result = segment_end;
 127   }
 128
 129   uint32 section_table_end = header_->e_shoff +
 130                              (header_->e_shnum * sizeof(Elf32_Shdr));
 131   if (section_table_end > result)
 132     result = section_table_end;
 133
 134   uint32 segment_table_end = header_->e_phoff +
 135                              (header_->e_phnum * sizeof(Elf32_Phdr));
 136   if (segment_table_end > result)
 137     result = segment_table_end;
 138
 139   return result;
 140 }
 141
 142 CheckBool DisassemblerElf32::IsValidRVA(RVA rva) const {
 143
 144   // It's valid if it's contained in any program segment
 145   for (int i = 0; i < ProgramSegmentHeaderCount(); i++) {
 146     const Elf32_Phdr *segment_header = ProgramSegmentHeader(i);
 147
 148     if (segment_header->p_type != PT_LOAD)
 149       continue;
 150
 151     Elf32_Addr begin = segment_header->p_vaddr;
 152     Elf32_Addr end = segment_header->p_vaddr + segment_header->p_memsz;
 153
 154     if (rva >= begin && rva < end)
 155       return true;
 156   }
 157
 158   return false;
 159 }
 160
 161 // Returns RVA for an in memory address, or NULL.
 162 CheckBool DisassemblerElf32::RVAToFileOffset(Elf32_Addr addr,
 163                                                 size_t* result) const {
 164
 165   for (int i = 0; i < ProgramSegmentHeaderCount(); i++) {
 166     Elf32_Addr begin = ProgramSegmentMemoryBegin(i);
 167     Elf32_Addr end = begin + ProgramSegmentMemorySize(i);
 168
 169     if (addr >= begin  && addr < end) {
 170       Elf32_Addr offset = addr - begin;
 171
 172       if (offset < ProgramSegmentFileSize(i)) {
 173         *result = ProgramSegmentFileOffset(i) + offset;
 174         return true;
 175       }
 176     }
 177   }
 178
 179   return false;
 180 }
 181
 182 RVA DisassemblerElf32::FileOffsetToRVA(size_t offset) const {
 183   // File offsets can be 64 bit values, but we are dealing with 32
 184   // bit executables and so only need to support 32bit file sizes.
 185   uint32 offset32 = (uint32)offset;
 186
 187   for (int i = 0; i < SectionHeaderCount(); i++) {
 188
 189     const Elf32_Shdr *section_header = SectionHeader(i);
 190
 191     // These can appear to have a size in the file, but don't.
 192     if (section_header->sh_type == SHT_NOBITS)
 193       continue;
 194
 195     Elf32_Off section_begin = section_header->sh_offset;
 196     Elf32_Off section_end = section_begin + section_header->sh_size;
 197
 198     if (offset32 >= section_begin && offset32 < section_end) {
 199       return section_header->sh_addr + (offset32 - section_begin);
 200     }
 201   }
 202
 203   return 0;
 204 }
 205
 206 CheckBool DisassemblerElf32::RVAsToOffsets(std::vector<RVA>* rvas,
 207                                            std::vector<size_t>* offsets) {
 208   offsets->clear();
 209
 210   for (std::vector<RVA>::iterator rva = rvas->begin();
 211        rva != rvas->end();
 212        rva++) {
 213
 214     size_t offset;
 215
 216     if (!RVAToFileOffset(*rva, &offset))
 217       return false;
 218
 219     offsets->push_back(offset);
 220   }
 221
 222   return true;
 223 }
 224
 225 CheckBool DisassemblerElf32::RVAsToOffsets(ScopedVector<TypedRVA>* rvas) {
 226   for (ScopedVector<TypedRVA>::iterator rva = rvas->begin();
 227        rva != rvas->end();
 228        rva++) {
 229
 230     size_t offset;
 231
 232     if (!RVAToFileOffset((*rva)->rva(), &offset))
 233       return false;
 234
 235     (*rva)->set_offset(offset);
 236   }
 237
 238   return true;
 239 }
 240
 241 CheckBool DisassemblerElf32::ParseFile(AssemblyProgram* program) {
 242   // Walk all the bytes in the file, whether or not in a section.
 243   uint32 file_offset = 0;
 244
 245   std::vector<size_t> abs_offsets;
 246
 247   if (!RVAsToOffsets(&abs32_locations_, &abs_offsets))
 248     return false;
 249
 250   if (!RVAsToOffsets(&rel32_locations_))
 251     return false;
 252
 253   std::vector<size_t>::iterator current_abs_offset = abs_offsets.begin();
 254   ScopedVector<TypedRVA>::iterator current_rel = rel32_locations_.begin();
 255
 256   std::vector<size_t>::iterator end_abs_offset = abs_offsets.end();
 257   ScopedVector<TypedRVA>::iterator end_rel = rel32_locations_.end();
 258
 259   for (int section_id = 0;
 260        section_id < SectionHeaderCount();
 261        section_id++) {
 262
 263     const Elf32_Shdr *section_header = SectionHeader(section_id);
 264
 265     if (section_header->sh_type == SHT_NOBITS)
 266       continue;
 267
 268     if (!ParseSimpleRegion(file_offset,
 269                            section_header->sh_offset,
 270                            program))
 271       return false;
 272     file_offset = section_header->sh_offset;
 273
 274     switch (section_header->sh_type) {
 275       case SHT_REL:
 276         if (!ParseRelocationSection(section_header, program))
 277           return false;
 278         file_offset = section_header->sh_offset + section_header->sh_size;
 279         break;
 280       case SHT_PROGBITS:
 281         if (!ParseProgbitsSection(section_header,
 282                                   &current_abs_offset, end_abs_offset,
 283                                   &current_rel, end_rel,
 284                                   program))
 285           return false;
 286         file_offset = section_header->sh_offset + section_header->sh_size;
 287         break;
 288       case SHT_INIT_ARRAY:
 289         // Fall through
 290       case SHT_FINI_ARRAY:
 291         while (current_abs_offset != end_abs_offset &&
 292                *current_abs_offset >= section_header->sh_offset &&
 293                *current_abs_offset <
 294                (section_header->sh_offset + section_header->sh_size)) {
 295           // Skip any abs_offsets appear in the unsupported INIT_ARRAY section
 296           VLOG(1) << "Skipping relocation entry for unsupported section: " <<
 297             section_header->sh_type;
 298           current_abs_offset++;
 299         }
 300         break;
 301       default:
 302         if (current_abs_offset != end_abs_offset &&
 303                *current_abs_offset >= section_header->sh_offset &&
 304                *current_abs_offset <
 305                (section_header->sh_offset + section_header->sh_size))
 306           VLOG(1) << "Relocation address in unrecognized ELF section: " << \
 307             section_header->sh_type;
 308       break;
 309     }
 310   }
 311
 312   // Rest of the file past the last section
 313   if (!ParseSimpleRegion(file_offset,
 314                          length(),
 315                          program))
 316     return false;
 317
 318   // Make certain we consume all of the relocations as expected
 319   return (current_abs_offset == end_abs_offset);
 320 }
 321
 322 CheckBool DisassemblerElf32::ParseProgbitsSection(
 323     const Elf32_Shdr *section_header,
 324     std::vector<size_t>::iterator* current_abs_offset,
 325     std::vector<size_t>::iterator end_abs_offset,
 326     ScopedVector<TypedRVA>::iterator* current_rel,
 327     ScopedVector<TypedRVA>::iterator end_rel,
 328     AssemblyProgram* program) {
 329
 330   // Walk all the bytes in the file, whether or not in a section.
 331   size_t file_offset = section_header->sh_offset;
 332   size_t section_end = section_header->sh_offset + section_header->sh_size;
 333
 334   Elf32_Addr origin = section_header->sh_addr;
 335   size_t origin_offset = section_header->sh_offset;
 336   if (!program->EmitOriginInstruction(origin))
 337     return false;
 338
 339   while (file_offset < section_end) {
 340
 341     if (*current_abs_offset != end_abs_offset &&
 342         file_offset > **current_abs_offset)
 343       return false;
 344
 345     while (*current_rel != end_rel &&
 346            file_offset > (**current_rel)->get_offset()) {
 347       (*current_rel)++;
 348     }
 349
 350     size_t next_relocation = section_end;
 351
 352     if (*current_abs_offset != end_abs_offset &&
 353         next_relocation > **current_abs_offset)
 354       next_relocation = **current_abs_offset;
 355
 356     // Rel offsets are heuristically derived, and might (incorrectly) overlap
 357     // an Abs value, or the end of the section, so +3 to make sure there is
 358     // room for the full 4 byte value.
 359     if (*current_rel != end_rel &&
 360         next_relocation > ((**current_rel)->get_offset() + 3))
 361       next_relocation = (**current_rel)->get_offset();
 362
 363     if (next_relocation > file_offset) {
 364       if (!ParseSimpleRegion(file_offset, next_relocation, program))
 365         return false;
 366
 367       file_offset = next_relocation;
 368       continue;
 369     }
 370
 371     if (*current_abs_offset != end_abs_offset &&
 372         file_offset == **current_abs_offset) {
 373
 374       const uint8* p = OffsetToPointer(file_offset);
 375       RVA target_rva = Read32LittleEndian(p);
 376
 377       if (!program->EmitAbs32(program->FindOrMakeAbs32Label(target_rva)))
 378         return false;
 379       file_offset += sizeof(RVA);
 380       (*current_abs_offset)++;
 381       continue;
 382     }
 383
 384     if (*current_rel != end_rel &&
 385         file_offset == (**current_rel)->get_offset()) {
 386
 387       uint32 relative_target = (**current_rel)->relative_target();
 388       // This cast is for 64 bit systems, and is only safe because we
 389       // are working on 32 bit executables.
 390       RVA target_rva = (RVA)(origin + (file_offset - origin_offset) +
 391                              relative_target);
 392
 393       if (! (**current_rel)->EmitInstruction(program, target_rva))
 394         return false;
 395       file_offset += (**current_rel)->op_size();
 396       (*current_rel)++;
 397       continue;
 398     }
 399   }
 400
 401   // Rest of the section (if any)
 402   return ParseSimpleRegion(file_offset, section_end, program);
 403 }
 404
 405 CheckBool DisassemblerElf32::ParseSimpleRegion(
 406     size_t start_file_offset,
 407     size_t end_file_offset,
 408     AssemblyProgram* program) {
 409   // Callers don't guarantee start < end
 410   if (start_file_offset >= end_file_offset) return true;
 411
 412   const size_t len = end_file_offset - start_file_offset;
 413
 414   if (!program->EmitBytesInstruction(OffsetToPointer(start_file_offset), len))
 415     return false;
 416
 417   return true;
 418 }
 419
 420 CheckBool DisassemblerElf32::ParseAbs32Relocs() {
 421   abs32_locations_.clear();
 422
 423   // Loop through sections for relocation sections
 424   for (int section_id = 0; section_id < SectionHeaderCount(); section_id++) {
 425     const Elf32_Shdr *section_header = SectionHeader(section_id);
 426
 427     if (section_header->sh_type == SHT_REL) {
 428
 429       Elf32_Rel *relocs_table = (Elf32_Rel *)SectionBody(section_id);
 430
 431       int relocs_table_count = section_header->sh_size /
 432                                section_header->sh_entsize;
 433
 434       // Elf32_Word relocation_section_id = section_header->sh_info;
 435
 436       // Loop through relocation objects in the relocation section
 437       for (int rel_id = 0; rel_id < relocs_table_count; rel_id++) {
 438         RVA rva;
 439
 440         // Quite a few of these conversions fail, and we simply skip
 441         // them, that's okay.
 442         if (RelToRVA(relocs_table[rel_id], &rva) && CheckSection(rva))
 443           abs32_locations_.push_back(rva);
 444       }
 445     }
 446   }
 447
 448   std::sort(abs32_locations_.begin(), abs32_locations_.end());
 449   return true;
 450 }
 451
 452 CheckBool DisassemblerElf32::CheckSection(RVA rva) {
 453   size_t offset;
 454
 455   if (!RVAToFileOffset(rva, &offset)) {
 456     return false;
 457   }
 458
 459   for (int section_id = 0;
 460        section_id < SectionHeaderCount();
 461        section_id++) {
 462
 463     const Elf32_Shdr *section_header = SectionHeader(section_id);
 464
 465     if (offset >= section_header->sh_offset &&
 466         offset < (section_header->sh_offset + section_header->sh_size)) {
 467       switch (section_header->sh_type) {
 468         case SHT_REL:
 469           // Fall-through
 470         case SHT_PROGBITS:
 471           return true;
 472       }
 473     }
 474   }
 475
 476   return false;
 477 }
 478
 479 CheckBool DisassemblerElf32::ParseRel32RelocsFromSections() {
 480
 481   rel32_locations_.clear();
 482
 483   // Loop through sections for relocation sections
 484   for (int section_id = 0;
 485        section_id < SectionHeaderCount();
 486        section_id++) {
 487
 488     const Elf32_Shdr *section_header = SectionHeader(section_id);
 489
 490     if (section_header->sh_type != SHT_PROGBITS)
 491       continue;
 492
 493     if (!ParseRel32RelocsFromSection(section_header))
 494       return false;
 495   }
 496
 497   std::sort(rel32_locations_.begin(),
 498             rel32_locations_.end(),
 499             TypedRVA::IsLessThan);
 500   return true;
 501 }
 502
 503 }  // namespace courgette